It has been long known that colored images can be obtained by a contact reaction of an electron donating or proton accepting colorless organic compound (hereinafter, referred to as a "color former") and an electron accepting or proton donating solid acid (hereinafter, referred to as a "developer"). Examples of the practical utilization of the foregoing phenomenon are pressure-sensitive copying papers as described in, for example, U.S. Pat. Nos. 2,505,470, 2,505,489, 2,550,471, 2,548,366, 2,712,507, 2,730,456, 2,730,457, 3,418,250 and 3,672,935 and heat-sensitive recording papers as described in, for example, Japanese Patent Publication Nos. 4160/68, 7600/68 and 14039/70 and U.S. Pat. No. 2,939,009.
Furthermore, a printing method for obtaining colored images by supplying an ink containing a color former to a sheet having a colored layer of a developer is described in, for example, West German Patent Application (OLS) No. 1,939,962.
The developer has the property defined above and examples include clays, phenol resins, metal salts of aromatic carboxylic acids, etc.
In general, such a developer is uniformly coated over the entire surface of a support and hence a method has been widely employed wherein portions of the developer sheet which are unnecessary for recording are desensitized by coating these portions with a composition containing a desensitizer using a printing machine, etc.
Desensitizers are described in, for example, U.S. Pat. No. 2,777,780, Japanese Patent Publication Nos. 27255/69, 21448/70, 22651/71 and 29546/71, Japanese Patent Application (OPI) No. 32915/72 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), Japanese Patent Publication Nos. 38201/72 and 4050/73, Japanese Patent Application (OPI) No. 6805/73, Japanese Patent Publication Nos. 4484/74, 19647/74, 23008/74 and 23850/74, Japanese Patent Application (OPI) Nos. 43708/74, 72009/74, 77709/74, 77710/74, 15513/74 and 83509/74, and West German Patent Application (OLS) Nos. 2,343,800, 2,359,079 and 2,361,856.
Specific examples of desensitizers are dodecyltrimethylammonium chloride, dodecylamine, 2,4,4-trimethyl-2-oxazoline, xylenediamine, polyoxyethylene alkylamine, polyoxyethylene alkyl ether, polyoxyethylene alkylphenyl ether, polyethylene glycol, polypropylene glycol, glycidyl ether addition products of amines, etc.
However, these desensitizers all have insufficient desensitizing effect and, in particular, are ineffective for diphenylmethane series color formers described in U.S. Pat. Nos. 3,193,404 and 3,278,327, Japanese Patent Publication No. 14873/61, Japanese Patent Application (OPI) Nos. 95402/73 and 148526/77, etc. That is, when the foregoing developer sheet coated with the desensitizer is brought into contact with a diphenylmethane series color former, the coated portions appear to be desensitized initially but colored images begin to appear with the passage of time. The desensitizing effect may be somewhat improved by increasing the amount of the desensitizer coated but in this case there is the disadvantage that when a colored ink is applied onto the surface of the desensitizer-coated portion by writing or printing, the written or printed image of the colored ink greatly fades or blurs.
Therefore, in spite of the features that the color formers are excellent in color density and the colored materials formed from the color formers are very stable as well as the cost of them is low, diphenylmethane series color formers cannot be used with conventional desensitizers, and hence the development of desensitizers showing good effect for diphenylmethane series color former has been strongly desired.